Fiber lasers provide significant advantages of efficiency and practicality in comparison with other laser types such as free-space lasers. In fiber lasers and amplifiers, light is guided by an “active” fiber core typically doped with ions of a rare-earth element, such as Ytterbium, which provides optical gain. The guiding property of the doped fiber core considerably relaxes requirements of optical alignment. It also allows one to increase the length of the gain medium to tens and even hundreds of meters, resulting in very high achievable optical gains.
With the advent of a double-clad fiber (DCF), fiber lasers have been scaled to kilowatt (kW) power levels. In a DCF, pump light propagates in a relatively large inner cladding, typically 125 to 600 micrometers in diameter, surrounding the doped core. The doped core has a much smaller diameter, e.g. 5 to 100 micrometers. The laser light propagates in the doped core. The inner cladding guides the pump light along the doped core, enabling the pump light to be efficiently absorbed in the doped core on the entire fiber length, causing laser light amplification to be distributed along the entire fiber length.
In a regime of high average power levels, fiber or other waveguide lasers and amplifiers may show a so-called modal instability. Modal instability may cause the laser light to be scattered into higher-order core modes and even cladding modes, thus causing a major degradation in either beam quality, usable power, or both. This instability has been studied in lasers generating sub-microsecond pulses at average powers of greater than about 100 W using large-mode-area fibers of various designs. By way of example, Eidam et al. in an article entitled “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers”, Optics Express, Vol. 19, Issue 14, pp. 13218-13224, 2011, describe a possible mechanism of a modal instability. This mechanism includes creating temperature gradients along the laser fiber due to interference of transversal lasing modes. The temperature gradients cause modulations of refractive index along the laser fiber, which in their turn increase energy coupling from a fundamental lasing mode into higher order transversal lasing modes, causing more modal interference, and accordingly more thermal variations along the laser fiber. Essentially, a runaway process develops, in which light energy is coupled out of fundamental lasing mode, degrading the laser beam quality and reducing the output optical power.
Various methods have been suggested to reduce modal instability in high power fiber lasers. For example, the entire length of the fiber laser may be actively temperature stabilized to counter the formation of the temperature gradients creating the modulations of refractive index, in an attempt to hold back the above described runaway process. Alternatively, a fiber laser cavity may be extended with temperature controlled portions, the optical length of which is dynamically adjusted to cause a destructive optical interference of higher-order modes, thus reducing a coefficient of cross-coupling between the fundamental and higher-order modes. However, in practice, these methods have not been successful in substantially suppressing modal instability.